Biomass fuel pellets are currently manufactured by extrusion through a rotating die and roller assembly, or by a pair of rotating wheels and a round cylindrical extrusion mould. Recently, a technique involving a block and reciprocating piston has been proposed. Common to all of these methods is production of a cylindrical pellet on the order of 2.5 cm long with an aspect ratio of about 8.
Such pellets are generally made of sawdust or similar biological material and dried to about 8-12% moisture content prior to densification. Torrefaction may be used to improve properties of the biomass as a fuel. Torrefaction typically involves slow heating of the biomass in an inert atmosphere to a maximum temperature of 300° C., yielding a solid uniform product with a lower moisture content and a higher energy content per unit volume compared to the initial biomass. The process removes undesirable components of the material while reducing weight and maintaining most of the original energy content.
The sides of cylindrical pellets are smooth as the lignin naturally found in wood is extracted and coats the surface; however, the ends of the pellets are rough and exposed. The exposed ends allow for absorption of moisture. Such pellets are prone to breaking from handling and storage, creating dust and fines which cause a dangerous environment at processing facilities.
Where torrefaction is not employed, pellets retain sufficient moisture and hydrocarbon volatiles as to make for problems co-firing in a power plant. Pellets made of untorrefied material are also energy intensive to grind and may cause gumming and mechanical issues with grinding equipment, thereby requiring modifications to processing technology in the power plant. Further, untorrefied pellets are of a lower specific energy density than torrefied pellets, so they are more expensive a fuel to use, based on transportation costs.